Colour & Light
- White light is a mixture of all the colours of the spectrum
- Each colour has a different wavelength (and frequency), making up a very narrow part of the electromagnetic spectrum
- White light may be separated into all its colours by passing it through a prism
- This is done by refraction
- Violet light is refracted the most, whilst red light is refracted the least
- This splits up the colours to form a spectrum
- This process is similar to how a rainbow is created
White light may be separated into all its colours by passing it through a prism
Differential Absorption of Colour
- Absorption occurs when:
Energy is transferred from the wave into the particles of a substance
- Waves can be partially or completely absorbed
- Sound waves are absorbed by brick or concrete in houses
- Light will be absorbed if the frequency of light matches the energy levels of the electrons
- The light will be absorbed, and then reemitted over time as heat
- If an object appears red, this means:
- Only red light has been reflected
- All the other frequencies of visible light have been absorbed
The object is seen as red since the red light is reflected whilst the other colours are absorbed
Transmission & Absorption – Colour Filters
- Colour filters work by absorbing certain wavelengths and transmitting other wavelengths
- These certain wavelengths correspond to certain colours
- When white light passes through a coloured filter, some colours are absorbed whilst others are able to pass straight through
- For example, when white light passes through a red filter:
- Red light is transmitted
- All the other colours are absorbed
- The colour that is transmitted is the same colour as the filter
A red filter transmits only red light and absorbs all other colours of light
Reflection
- The colour of an opaque object is determined by which wavelengths of light are more strongly reflected
- Wavelengths that are not reflected are absorbed
- Hence, this is why different objects appear to be different colours
- For example, white light upon a green surface will only have green light reflected and the others absorbed
- This light is reflected into our eyes to see the surface in that colour
A green surface reflects green light and absorbs all other colours
- An object will appear white if:
- All wavelengths are reflected equally
- An object will appear black if:
- All wavelengths are absorbed
- An object will appear transparent if:
- All the light is transmitted, and only a small amount is reflected or absorbed
Specular Reflection
- Specular reflection is defined as:
Reflection from a smooth surface in a single direction
- When light reflects off a smooth surface, such as a mirror, specular reflection occurs
- This is what gives a mirror its shiny appearance
- This is why a reflection can be seen clearly in a mirror
- In this case, the angle of reflection r is equal to the angle of incidence i
When reflecting off a mirror, the specular reflection occurs. The angle of incidence is equal to the angle of reflection
Scattering
- Scattering is defined as:
Reflection from a rough surface in many directions
- When light reflects off a rough surface, which applies to the majority of surfaces, diffuse reflection occurs
- This is what gives objects a dull or matt appearance
- This is why a reflection cannot be seen clearly from a table surface, for example
- Even though a table's surface may look smooth from afar, it is actually made up of many tiny ridges which the light rays are scattered off
- When light scatters, it leaves the surface in all directions
When light is reflected from the majority of surfaces it is scattered – a process known as diffuse reflection
